The cell forming the subject matter of the invention can be associated with various known characterisation means such as:                laser-induced fluorescence;        fluorescence spectrometry;        absorption spectrometry;        Raman spectrometry;        infrared spectrometry.        
By way of non-limiting example, the following description is based on the use of laser-induced breakdown spectrometry, or LIBS analysis. This method consists in focussing a pulsed laser beam into a reactional mixture to be analysed and forming plasma which is analysed by emission spectrometry. This determines the composition of said reactional mixture. This technique is applied in the description hereinbelow to control of smoke coming from the production of nanometric particles by laser pyrolysis.
A LIBS system for LIBS analysis is illustrated in FIG. 1 and comprises a nanoparticle synthesis reactor A5, a LIBS cell A1, a laser A2 for emitting a laser beam, a lens A3 for converging the laser beam inside the LIBS cell A1, an optical system A4 for collecting signals coming from the LIBS cell A1, and a spectrometer A7.
Production of nanometric particles within the reactor A5 is based on the interaction of crossed flows between a beam emitted by a laser, for example a carbon dioxide CO2 power laser, and a reactional mixture. The beam excites vibrational states of the molecules (so-called precursors) of the reactional mixture. The energy transmitted from the beam to the molecules is redistributed to the entire reactional mixture by collision. There is then very rapid elevation of the temperature of the reactional mixture which causes thermal decomposition of the molecules, resulting in a so-called “supersaturated” vapour in radicals and in energy. Nanoparticles then form from the radicals by homogeneous germination. The nanoparticles grow by a phenomenon of collision/coalescence growth.
Dissociation and formation of nanoparticles take place in a overlapping volume between the beam and the flow of the reactional mixture observable by way of the production of a flame at this point.
When the nanoparticles exit from this volume, they undergo a quenching effect which stops their growth.
The nanoparticles are then guided to the LIBS cell A1 through an entry conduit A6.
The LIBS cell A1 comprises a reaction chamber and four arms:                a first arm A11 forming inlet orifice A111 for the smoke;        a second arm A12 facing the first and forming outlet orifice A121 for evacuating of smoke;        a third arm A13 closed by a window A131 through which the laser beam intended to form a plasma enters; and        a fourth arm A14 closed by a cache A141 and facing the third arm A13 is not used.        
The LIBS cell A1 also comprises a viewing window A15 for observing the plasma with the naked eye.
In the LIBS cell A1, the nanoparticles behave as a gas and therefore expand inside the reaction chamber and occupy all the space available and form, smoke.
Inside the reaction chamber, the laser beam Flaser generated by the laser A2 is focussed by the lens A3. When the laser beam Flaser is focussed in the mixture to be analysed there is vaporisation of nanoparticles causing ejection of atoms and forming plasma which expands. During expansion of the plasma, atoms de-energise, causing the emission of light. This light is then received by the optical system A4 which is adapted and placed to the same side as the laser A2. This light is then analysed by the spectrometer A7 connected to the optical system A4 via fibre optics A8 adapted to transport the signal.
On drawback of this LIBS cell results from the fact that the nanoparticles behave as a gas within the reaction chamber. This is why the analysis window A131 of the third arm A13 becomes clogged. The clogged analysis window A131 acts as a filter which blocks part of the laser beam Flaser. Not all the energy of the laser beam Flaser is therefore efficient and only a portion thereof can be used to form the plasma. The formed plasma is therefore less energetic and emits a lower signal. This already weakened signal is further attenuated when it passes back through the analysis window A131 of the third arm towards the optical system A4.
Another drawback, still linked to the gaseous behaviour of the smoke, is the clogging of the viewing window A15 tending to obstruct observation of the plasma with the naked eye.
Yet another drawback is that the formed plasma is not limited to the focal point of the laser beam Flaser, that is to say where the latter is the most highly concentrated. In fact, as particles are present throughout the reaction chamber, secondary plasmas Plsec can form between the focal point of the laser beam, where the main plasma Plpr forms, and the analysis window A131 through which the laser beam Flaser enters the reaction chamber, as illustrated in FIG. 2. The secondary plasmas Plsec can be located outside the observation zone by the optical system A4.
Another drawback of the LIBS cell A1 hereinabove is instability of the signals acquired by the optical system A4 and by the spectrometer A7, the origin of which is numerous.